1. Field of the Invention
The present invention relates to an image forming apparatus transferring a toner image on a transfer sheet, and in particular, to improvement of an electrostatic charge eliminator for discharge of the transfer sheet after transfer.
2. Description of the Related Art
Image forming apparatuses normally have a basic configuration consisting of a photosensitive drum (image bearing member) driven to rotate around its shaft center, and an electrostatic charger, an exposure device, a developing device, a transfer member and a cleaning device provided around it.
The photosensitive drum is driven to revolve around its shaft center, and the peripheral surface of the photosensitive drum is charged uniformly by application of high voltage from the electrostatic charger. An electrostatic latent image is then formed on the peripheral surface by irradiation of the light emitted from the exposure device, based on the image information, onto the peripheral surface of the uniformly charged photosensitive drum. Toner particles are then supplied from the developing device onto the peripheral surface of the photosensitive drum carrying the electrostatic latent image forming a toner image corresponding to the electrostatic latent image on the same peripheral surface. The toner image is charged, for example, positively.
The toner image formed on the peripheral surface of photosensitive drum as it is charged positively reaches the position of the transfer member by revolution of the photosensitive drum. When the transfer sheet is fed into a nip portion between the photosensitive drum and the transfer member, negative electric charge is applied to the transfer sheet from the transfer member, and the toner image is transferred onto the transfer sheet by the electrostatic attractive force. The transfer sheet after transfer is fixed under heat in the fixing device and discharged out of the apparatus.
The transfer sheet after transfer processing occasionally adhere to the revolving photosensitive drum, because the toner image may not be separated easily from the peripheral surface of the photosensitive drum, which leads to a trouble of the transfer sheet not being conveyed to the fixing device. Such a trouble, which often occurs under low-temperature and low-humidity environment, leads also to staining of the transfer sheet.
To prevent such a trouble, the image forming apparatus disclosed in Japanese Patent Unexamined Publication No. 4-12382 (D1) has charge-eliminating teeth that may be brought into the electrically floating state (electrically floating neutral state without grounding or voltage application) at a position close to the region between the photosensitive drum and the transfer member facing each other. The apparatus disclosed in D1 has an additional control means of controlling the electric current supplied to the transfer member, and conducts the transfer processing smoothly by properly controlling the state of the charge-eliminating teeth and the electric current supplied to the transfer member.
In addition, the image forming apparatus disclosed in Japanese Patent Unexamined Publication No. 6-194966 (D2) has charge-eliminating teeth with the edge facing the position between the photosensitive drum and the transfer member facing each other, and thus, improves the action by the charge-eliminating teeth of separating the toner image form the photosensitive drum.
Further, the image forming apparatus disclosed in Japanese Patent Unexamined Publication No. 10-282799 (D3) has charge-eliminating teeth with the edge facing the position between a photosensitive drum and a transfer member facing each other and a barrier of an insulating material provided between the transfer member and the charge-eliminating teeth. In the apparatus disclosed in D3, short circuiting between the charge-eliminating teeth and the transfer member is prevented by the barrier, and thus, the toner image-separating efficiency of the charge-eliminating teeth is improved.
However, the apparatus disclosed in D1 demands an additional control means of controlling electric current, and thus, has a problem that the costs becomes higher.
Also in the apparatus disclosed in D2, the charge-eliminating teeth are merely provided at a position facing the position between the transfer member and the photosensitive drum facing each other. Thus, such an apparatus has a problem that the electric field formed by the charge-eliminating teeth does not always function properly to separate the toner image form the photosensitive drum peripheral surface, and the electric field does not show a desirable action because of leakage to the transfer member or the photosensitive drum.
Further, the apparatus disclosed in D3, in which a barrier of an insulating material is merely placed between the charge-eliminating teeth and the transfer member, does not give an advantageous effect as much as expected.